Fuel Indices for Estimation of Slagging of Phosphorus-Poor Biomass in Fixed Bed Combustion

Abstract

The market for solid biofuels will grow rapidly during the coming years, and there will be a great demand for raw materials. This will force the existing fuel base to also cover wooden materials of lower qualities as well as agricultural raw materials and residues, which often show unfavorable ash-melting temperatures. This may lead to combustion-related problems. Thus, for the utilization of lower quality fuels, it is important to be able to predict potential fuel ash-related problems such as slagging. In light of this, the first objective of the present paper was to evaluate the applicability of previously defined indices for slagging of biomass fuels (phosphorus-poor) in fixed bed combustion. The evaluation showed that none of the previously suggested indices in the literature are suitable for qualitative (nor quantitative) prediction of slagging during fixed bed combustion of P-poor biomass fuels. Hence, a second objective was to develop improved novel fuel indices that can be applied to estimate the slagging of phosphorus-poor biomass in fixed bed combustion. The novel fuel indices give a qualitative prediction of the slagging tendency in biomass fixed bed combustion but still needs additional work to further extend the compositional range as well as to fine tune the indices’ boundaries.

Understanding the effects of oxyfuel combustion and furnace scale on biomass ash deposition

“…This suggests that the recycled wood ash belongs to the ash type with less ash deposition issues than the ash types with the high potassium level according to the ash classification method proposed by Vassilev et al [16,17], which is based on the relationship between the ash fusion behaviour and ash composition. In addition, a similar method proposed by Nazelius et al [18] indicates that this kind of ash type belongs to the low-medium slagging ash for the fixed bed combustion condition. However, under the pulverised fuel combustion condition, in addition to the overall ash fusion behaviour, the ash deposit formation is dictated by the particle size based on the ash chemistry, physical property (size, density, etc.…”

Does Mechanical Screening of Contaminated Forest Fuels Improve Ash Chemistry for Thermal Conversion?

“…From the fuels’ ash composition (in wt % of oxides), the prediction of slagging tendencies for biomass fuels that are poor in P can be presented in a compositional diagram consisting of K 2 O + Na 2 O, CaO + MgO, and SiO 2 (Figure ). The consolidation of K and Na in the ternary diagram is based on the assumption that, in the combustion of lignocellulosic biomass, Na content is generally low and can be assumed to have similar behavior as K, and thus, the concentrations of K and Na can be combined. The same approximation can be made for Ca and Mg. By interpretation of the compositional diagram, it is shown that all fuel assortments have a composition resulting in higher slagging potential (black-colored area = major slagging tendency).…”

“…However, extrinsic Si has previously been shown to be less reactive than inherent Si, , so with a high content of extrinsic Si from soil contaminants, a slightly lower slagging tendency could be expected than the phase diagram indicates. On the other hand, Näzelius et al found sintered ash after combustion of fuels containing contaminants despite their relatively low slagging tendency according to the fraction of fuel ash that forms slag. Lindström et al suggested that contaminants might increase the Si concentration in the melt by dissolving into the molten ash.…”

Demonstrating Fuel Design To Reduce Particulate Emissions and Control Slagging in Industrial-Scale Grate Combustion of Woody Biomass

“…This approach has been successfully applied to various types of biomasses and demonstrated in small- and lab-scale burners. ,− Successful applications of fuel design rely on understanding which ash transformation reactions are desired based on the fuel composition. An example of the Supporting Information to determine appropriate fuel ash composition has been presented for predicting the risk and severity of slag formation in the combustion of phosphorus-poor biomass, based on the system (K 2 O + Na 2 O)–(CaO + MgO)–SiO 2 , as shown in Figure . It is important to demonstrate and validate the fuel design concept on an industrial scale in a variety of combustion systems to gain acceptance in the bioenergy sector, thereby enabling a broader fuel feedstock by utilizing existing knowledge of ash transformation reactions without requiring large investments in existing infrastructure.…”

“…The liquid isotherms are adopted from the K 2 O–CaO–SiO 2 system in the work of Morey et al, later revised by Roedder . Images adapted with permission from Näzelius et al…”